Temperature stable ultrasonic delay lines



1964 H. HOOVER ETAL TEMPERATURE STABLE ULTRASONIC DELAY LINES Filed June 19. 1961 IUZ.1

so \WB 5 IO I5 20 25 INVENTORS A/ER6ER7' Z. #aaVL-E 4ND firmer/1y ti A/ORDBE/FG United States Patent ice 3,154,425

3,154,425 TEMPERATURE STABLE ULTRASONIC DELAY LS Herbert 1.. Hoover and Martin E. Nordberg, Corning,

N .Y., assignors to Corning Glass Works, Corning, N.Y.,

a corporation of New York Filed June 1?, 1961, Ser. No. 118,185 11 Claims. (Cl. 106-53) This invention relates to solid ultrasonic delay lines, and more particularly to an ultrasonic delay line embodying a glass delay medium that is temperature stable.

One application for ultrasonic delay lines is in the radar receiver art, for example in moving target indication (MTI) devices. In such a device, the delay line is used to delay one received echo or pulse for comparison with a succeeding echo or pulse in order to determine whether any change has occurred in the time of the received signal. Another application is to provide for information storage in digital computers.

Since the inception of solid ultrasonic delay lines for these and other applications, vitreous silica has enjoyed widespread use as the delay, or wave propagation, medium in such delay lines. Materials having a crystalline structure have been avoided essentially because of excessive acoustic loss due to scattering. In general, organic matrials are characterized by a high degree of attenuation, that is, loss of signal amplitude during transmission through the delay medium. Large single crystals have been seriously considered but are undesirable because of the directional characteristic of their elastic properties. Among glasses, vitreous silica has been of particular interest because of its extremely low attenuation characteristic.

It has been observed, however, that this silica medium is characterized by a negative temperature coefficient of time delay for shear waves that is on the order of 80 parts per million per centigrade degree. It also has a negative temperature coefficient of acoustic attenuation that is on the order of 5 to per centigrade degree. A co-pending application, S.N. 50,366, filed August 18, 1960, in the name of Jason H. Eveleth and having a common assignee, describes the adverse eifects resulting from large temperature coefiicients and one means of overcoming such effects.

In general, it has been necessary to package delay lines in temperature controlled cases, usually containing a heating element, to counteract the effects of significant temperature changes. The expense and inconvenience of such auxiliary packaging and heating equipment is undesirable in any application, but particularly so in mobile and/ or compact equipment such as air-borne radar, computers, and the like.

Ideally, the temperature coefficient of time delay for a delay medium should approximate zero. However, small variations can be tolerated; thus glasses having temperature coeflicients up to about 8 parts per million per centigrade degree in absolute magnitude are generally acceptable.

It is also important that the material have as low an attenuation as possible, preferably not over about 8 10 decibels per cycle as measured at frequencies above one megacycle per second. The attenuation in fused silicas may be as low as 1 to 2 l0 decibels per cycle. However, most commercially available multi-component glasses have attenuations on the order of 10 to 60x10- decibels per cycle, which is much higher than desirable.

It is also desirable that a material for delay line media have a relatively low temperature coeflicient of attenuation, and it retain a stable characteristic delay time on aging. The latter is referred to as temporal stability of delay time and usually requires that the life-time change Patented Oct. 27, 1964 not exceed about 0.003%. For example, this is equivalent to about 0.001 microsecond in a thirty microsecond delay line.

It is a primary purpose of the present invention to provide improved solid ultrasonic delay lines employing glass as the delay medium. A further purpose is to obviate the deficiencies encountered in lines employing vitreous silica as the delay medium. A further purpose is to provide glass delay media having the acceptable acoustical characteristics outlined above. A specific purpose is to provide a delay line incorporating an alkali-lead-silicate glass as the time delay medium.

In accordance with our invention, a solid ultrasonic delay line comprises a delay medium composed of an alkali-lead-silicate glass, the composition of which consists essentially of heavy metal oxides equivalent to 2050% PbO, alkali metal oxides equivalent to 420% K 0, and the remainder SiO In a preferred embodiment, the glass is essentially composed of K 0, PhD and SiO but, under certain circumstances, other oxides may be considered equivalents, and substituted therefor, as described in detail later.

The invention is further described with reference to the accompanying drawing, in which:

FIG. 1 is a schematic illustration of a delay line assembly, and

FIG. 2 is a graphical illustration of a family of glasses having special utility in connection with the present invention.

A simple delay line assembly, as schematically illustrated in FIG. 1, includes a delay or transmission medium 10, transducer members 11 and 12, backing members 13 and 14, and electrical signal input and output circuits 15 and 16. Delay medium 10 may be of any conventional form such as an elongated cylinder, a disc, a polygonal plate or other known delay line medium geometry. It is molded from an alkali-lead-silicate glass, as hereafter described, and ground and polished in accordance with usual glass delay medium finishing procedures.

Transducers 11 and 12 are composed of a crystalline piezo-electric material, such as barium titanate or crystalline quartz. They are sealed to, or otherwise maintained in tight contact with, facets on the delay medium 10. Backing members 13 and 14 are firmly attached to crystals 11 and 12, respectively. Circuits 15 and 16, shown diagrammatically, may include any of the various components that are conventionally associated with delay lines. A schematic illustration is employed inasmuch as the improvement provided by this invention relates specifically to the delay medium 10.

FIG. 2 graphically illustrates a family of glasses which our research has shown to be particularly advantageous for the fabrication of delay lines. In this figure, the content of PhD in percent by weight is plotted along the vertical axis. The corresponding percentage by weight of K 0 is plotted along the horizontal axis. With the PbO and K 0 content for any glass thus fixed, the SiO content is determined by dilierence, viz., (percent PbO-l-percent K 0). In general, any glass falling within the rhombic area designated A will possess the satisfactory delay line properties outlined earlier. In other words, the glasses will have temperature coefficients of time delay not exceeding about 8 parts per million per centigrade degree over the temperature range of 40 C. to 100 (3.; their signal attenuation will not exceed about 8x10 decibels per cycle; temporal stability will not exceed about 0.003%; and the temperature coeflicient of attenuation will be relatively low. Line B,

traversing rhombic area A, represents glass compositions having essentially a zero temperature coeflicient of time delay. These are preferred compositions in applications where maximum temperature stability is of prime importance.

We have further found that the PbO content of glasses as defined with reference to FIG. 2 may be partially substituted for by other heavy metal divalent oxides, in particular CdO and BaO, in amounts up to about 10%. Also, Bi O may be substituted in amounts up to about 20%. These substituents, in the amounts indicated, tend to decrease attenuation slightly and, in the glasses of higher lead content, slightly increase temporal stability. They tend to increase temperature coefficient of time delay, but this can be compensated for by an adjustment in the alkali content. In amounts above the indicated ranges, they have a generally adverse effect on acoustical properties. In general, other substituents or additives provide no acoustic advantage and may have an adverse effect, particularly with respect to attenuation. However, minor amounts of antimony oxide, as a fining agent, and other compatible glass forming oxides, such as boric oxide, may be used as glass melting aids.

Our studies have further shown that the degree of acoustic attenuation in a glass decreases as K is replaced by Na O, particularly in glasses having a high total alkali metal oxide content. A minimum value is reached when about one quarter of the total alkali metal oxide content, by weight, is Na O, that is the actual ratio of K ocNa O, by weight, is about 3:1. Thereafter the trend changes direction, that is the degree of attenuation becomes greater as the Na O content in a glass is further increased. It exceeds the limit of 8 10- decibels/ cycle when the ratio of Na O:K 0 exceeds about 1, and, accordingly, such further substitution is undesirable.

Replacement of K 0 with Na O, and resultant formation of a mixed alkali glass, also increases temporal instability. This effect also peaks with increasing Na O replacement, in this case with about /3 to b of the K 0 replaced. The effect on temporal instability is more pronounced in glasses of high total alkali metal oxide content, and hence, mixed alkali is avoided in such glasses especially where stability is an important factor. It may be noted that similar effects are noted in substituting other alkali metal oxides for K 0, e.g. Na O and Li O in combination.

In general, substitution of Na O for K 0 without changing temperature coefficient may be on the basis of about 0.411 on a weight basis, that is about 0.4% of the former for 1% of the latter with SiO also being adjusted to account for the difference. Correspondingly, Li O may be substituted on the basis of about 0.3% by weight for 1% K 0. Rb O may be substituted on the basis of about 2.4% Rb O for 1% K 0, but is of little interest, in part because of economic considerations. The minimizing effect of such substitutions on attenuation permits an extension of the useful glass area to include the area designated by C in FIG. 2. In terms of percent by weight, the alkali metal oxide content equivalent to K 0 may be 4 to 20%, the PhD (or equivalent) content 20 to 50% and the remainder essentially silica. The compositions will further correspond to the areas designated A and C in FIG. 2 with the K 0 and PbO contents taken as K 0 and PbO equivalents respectively. With less than 20% PbO, these glasses tend to lack chemical stability, and above about 50% PbO, their attenuation becomes too high.

Line B in FIG. 2 may be mathematically expressed by the formula:

Rb O 2.4.

Pb0= 62- (K2o+2.5 Na2O+ 3 LizO+ 4 the permissible PbO content, within the limits 20 to 50%, may be expressed as 62i2 (K,o+2.5 Na O+3 LizO+ fl Alternatively, the alkali metal oxide content, in terms of K 0 equivalent, may be expressed as the PbO being within the range 20 to 50%.

The present glass compositions are similar in type to known optical and art glasses. Accordingly, they may be melted in accordance with known methods of melting such prior glasses. Thus, they may be melted in a continuous optical glass melting unit delivering about 1200 lbs. per day. The melting temperature may be 1200- 1400 C. and usual precautions as to purity in batch materials and melting practice will be observed.

The molten glass may be shaped into blanks having roughly the desired form and subsequently finished by the usual grinding and polishing procedures. Large shaped blanks may, of course, be cut to any desired shape. The blanks may be annealed in accordance with conventional optical glass practice. However, it has been found that temporal stability is markedly improved by a slow annealing rate to temperatures well below the usual minimum annealing temperature, that is the strain point of the glass. 7

The following table sets forth, by way of more specifically illustrating the invention, the compositions of certain glasses, in percent by weight as calculated from the batch composition, together with acoustic properties measured for these glasses:

Table SiO; 47 53 58. 4 53 60 57. 8 57. 8 58 PbO 34 30 20 10 18.7 K 0 3.2 5 10 10 14 NagO 1.3 4 2.2 2.2 Li2O 0.5 1 BaO 8.0 CdO 9.3 Blgoan 10 20 T.C.D. (X 10' /C. 0 0 0 3 5 8.1 5.9 a (X 10- db/cy.) 6 4 6 5.5 5 6.7 7.5 7. 6 AD, (percent X 10- 0.3 0.6 36 33 In this table temperature coefficient of time delay is measured in parts per centigrade degree, and identified as T.C.D. Likewise, a identifies attenuation in decibels per cycle and AD, temporal instability in percent.

Glasses of Examples 1 and 2 are particularly suitable because of their low temporal instability as well as low attenuation and temperature coefiicients. Examples 3 and 6 illustrate glasses having desirable properties except for temporal instability. Stability data for Examples 4, 5, 7, and 8 are not available. However, Examples 4 and 8 might be predicted to be acceptable, while Examples 5 and 7 would not be, owing to high Na O contents. Each of the compositions omits a fining agent, and the addition of 0.5% Sb O is satisfactory for this purpose.

For present purposes, the temperature coefficient of time delay (T.C.D.) is an average value generally expressed as,

L X0 Tr-T T and T are temperatures in C. at the extremes of a given temperature range, X and X are the time delays for the delay medium measured, with the medium at the corresponding temperature extremes, in microseconds, and X is the time delay at an intermediate temperature.

In measuring values of T.C.D. for glasses, a rectangular bar of the glass, 4" x 1" x 4", is cut and carefully ground and polished. The bar is provided with parallel end facets, to which are sealed, eg with phenyl benzoate,

silica bars having piezoelectric transducers sealed to their ends. A shear wave is then propagated through the glass test bar in 20 megacycle per second pulses of one microsecond duration, and measurements of time delay are made at precisely regulated temperatures. In evaluating the present glass media, measurements were made over a range of C. to +60 C.

Similar glass test specimens were employed for attenuation coefiicient measurements. As indicated earlier, attenuation is an expression of the decrease in, or loss of, amplitude of an acoustical wave or vibration as it is propagated through a delay medium. In determining coeflicient of attenuation, designated by oz, the relative decrease of the amplitude of the input and output signals is measured; this figure is divided by the product of the measured time of propagation through the bar and the frequency of the acoustical vibration to provide the characteristic attenuation per wave length of wave traverse.

Total attenuation is expressed as where a is the attenuation coefiicient, f is the vibration frequency of the propagated wave and t is the total delay time of the medium in microseconds. In turn the atwhere amplitude, is the wave amplitude at a given point and amplitude is the wave amplitude at a point spaced one wave length along the axis of propagation. The coeflicient then is in units of decibels per cycle.

Temporal instability, AD,,, is the percent change in the characteristic delay time of a particular medium over some period of time. Usually this is specified in terms of an indefinitely long time and referred to as lifetime stability.

What is claimed is:

1. In a solid ultrasonic delay line, an alkali-lead-silicate glass delay line medium having a temperature coefiicient of time delay not exceeding eight parts per million per C., an attenuation not exceeding 8 10 decibels per cycle, and being composed of about 10-50% PbO, 010% CdO, 0-10% BaO, 0-20% Bi O the total content of PbO, CdO, BaO and Bi O being about -50%, about 420% K 0 in which up to of the K 0 may be substituted by at least 1 additional alkali metal oxide in the indicated amount selected from the group consisting of 04% Na O, 03% U 0, and 0-24% Rb O, and wherein the substitution is on the basis of 0.4 part Na O for one part K 0, 0.3 part Li O for 1 part K 0 and 2.4 parts Rb O for 1 part K 0, the content of recited heavy metal oxides varying in inverse manner with the content of 6 recited alkali metal oxides, and the balance of the composition being essentially silica.

2. A delay line in accordance with claim 1, wherein the composition of the delay medium falls within a rhombic area designated by A and C in FIG. 2 of the drawing.

3. A delay line in accordance with claim 1, wherein the delay medium has temperature coefficient of time delay that is essentially zero and a composition falling on line B in FIG. 2.

4. A delay line in accordance with claim 1, wherein the delay medium has a lifetime temporal instability of not over 0.003%, consists essentially of K 0, PbO and SiO and has a composition within area A in FIG. 2.

5. A delay line in accordance with claim '1 wherein the composition of the delay medium includes up to 10% CdO.

6. A delay line in accordance with claim 1 wherein the composition of the delay medium includes up to 10% BaO.

7. A delay line in accordance with claim 1 wherein the composition of the delay medium includes up to 20% Bi O 8. A delay line in accordance with claim 1 wherein the composition of the delay medium includes at least two alkali metal oxides at least one of which is K 0 in an amount not less than one half the total alkali metal oxide content.

9. A delay line in accordance with claim 8 wherein the K 0 content is about three times the remaining alkali metal oxide content.

10. A delay line in accordance with claim 1 wherein the composition of the delay line medium is such that the content of recited heavy metal oxides within the range range of 2050% is equal to Rb O 2.4

11. A delay line in accordance with claim 1 wherein the composition of the delay line medium is such that the content of recited heavy metal oxides within the range of 20-50% is equal to (62 42 minus 175(K2O+2.5 Na2O+3 Li O+ RbgO 2.4

62 minus 15 7 K2O+2.5 Na O+3 LizO-l- References Cited in the file of this patent UNITED STATES PATENTS 

1. IN A SOLID ULTRASONIC DELAY LINE, AN ALKALI-LEAD-SILICATE GLASS DELAY LINE MEDIUM HAVING A TEMPERATURE COEFFICIENT OF TIME DELAY NOT EXCEEDING EIGHT PARTS PER MILLION PER *C., AN ATTENUATION NOT EXCEEDING 8X10**-3 DECIBELS PER CYCLE, AND BEING COMPOSED OF ABOUT 10-50% PBO, 0-10% CDO, 0-10% BAO, 0-20% BI2O3, THE TOTAL CONTENT OF PBO, CDO, BAO AND BI2O3 BEING ABOUT 20-50%, ABOUT 4-20% K2O IN WHICH UP TO 1/2 OF THE K2O MAY BE SUBSTITUTED BY AT LEAST 1 ADDITIONAL ALKALI METAL OXIDE IN THE INDICATED AMOUNT SELECTED FROM THE GROUP CONSISTING OF 0-4% NA2O, 0-3% LI2O, AND 0-24% RB2O, AND WHEREIN THE SUBSTITUTION IS ON THE BASIS OF 0.4 PART NA2O FOR ONE PART K2O, 0.3 PART LI2O FOR 1 PART K2O AND 2.4 PARTS RB2O FOR 1 PART K2O, THE CONTENT OF RECITED HEAVY METAL OXIDES VARYING IN INVERSE MANNER WITH THE CONTENT OF RECITED ALKALI METAL OXIDES, AND THE BALANCE OF THE COMPOSITION BEING ESSENTIALLY SILICA. 